Multiple wavelength optical isolator

ABSTRACT

A multiple wavelength optical isolator provides isolation for the light characteristic of either a neodymium glass laser, 1.06 Mu m., or ruby, 0.6943 Mu m. by immersion in either liquid nitrogen or the combination of dry ice and acetone respectively. The isolator includes a piece of silicate glass doped with 58 weight percent of terbium oxide which is subjected to the field of a permanent magnet of approximately 1100 gauss parallel to the path of light in the glass. The thickness of glass in this dimension is approximately 7.7 cm.

inventor Appl. No. Filed Patented Assignee MULTIPLE WAVELENGTH OPTICAL ISOLATOR 2 Claims, 1 Drawing Fig.

SEARCH ROOM Primary Examiner-David Schonberg Assistant Examiner-Paul R. Miller Attorneys-William C. Nealon, Noble S. Williams and Robert .1. Bird ABSTRACT: A multiple wavelength optical isolator provides isolation for the light characteristic of either a neodymium glass laser, 1.06 p.m., or ruby, 0.6943 pm. by immersion in either liquid nitrogen or the combination of dry ice and US. 350/151, acetone respectively. The isolator includes a piece of silicate 350/147 glass doped with 58 weight percent of terbium oxide which is Int. G02! l/22 subjected to the field of a pennanent magnet of approximately Field of Search 350/147, 1100 gauss parallel to the path of light in the glass. The

150, 151, 160 thickness of glass in this dimension is approximately 7.7 cm

1 lo I I I 120 I4 F f t a i I 1 2 i s I i 1' l 22 H PATENTED AUG3I I97| AGEN MULTIPLE WAVELENGTH OPTICAL ISOLATOR lt is well known that when a beam of plainzpolarized light passes through a plate or ordinary glass which is sabjected to a magnetic field with the lines of force of the field in the same direction as the light direction, the plane of polarization of this light is rotated somewhat due to what is commonly called a Faraday magnetooptic effect. Various types of doped silicate glasses have been used as Faraday rotatorsbut they have not functioned as well as might be desired for they have failed to provided as high Verdet constants as may be desired. (V=0.071 at 700 m., for example). The silicate glasses heavily doped with lead have high absorption rates for transmitted light and additionally, since they are not paramagnetic, they do not increase their Verdet constant with a decrease in temperature. Metaphosphateglass, containing rare earth metal oxides, is another known Faraday rotator, but these glasses have exhibited only a fair value for Verdet constant. Certain other materials exhibit large Verdet constant values but these have shown to have poor light transmission and cannot, therefore, be used" in a thickness sufficient to give a rotation of desired magnitude. The present-invention providesan improvement in Faraday rotation means, which have large -Verdet constants for polarized light and at the same time provides good light transmission characteristics for unpolarized light of selected wavelengths within predetermined wavelength regions of the light spectrum so as to be useful in optical systems for allowing or preventing the passage of, or for variously controlling the rotation of, the plane of polarization of polarized light passing therethrough. The improvement provides a Faraday rotation in glass formed of alumina-silicate glassdoped with high percentages of terbium, to provide a large Verdetconstant with good light transmission at preselected wavelengths, and additionally providing properties with other desired optical and physical properties such as high refractive index with low dispersion, good workability during fabrication, good stability at room temperature, and good resistance to chemical attack and the like. The improvement specifically provides a multiple wavelength optical-isolator, whereby with a given permanent magnet a single optical isolator may be useful at two or more of the commonly used laser wavelengths of 1.06 microns and 6,943 angstroms. The glass is cut to predetermined dimensions, and when :immersed in a coolant, at a predetermined temperature, a particular predetermined rotation is achieved. For example, a terbium-doped glass having a length of 8.2 cm. under the influence of the field of a permanent magnet immersed in liquid nitrogen provides a rotation of 45 with light of 1.06 microns wavelength, but the same glass and magnet with light of a wavelength of 0.6943 microns and immersed in acetone anddry ice produces a rotation of 5 1 .5.

It is, therefore, a principal-object of the invention to provide a multiple wavelength optical isolator, which produces as a single optical isolator; variable predetermined rotation of polarized light beams at predetermined, easily attainable, temperatures of operation. I

Another object of this invention is to provide the required axial magnetic field with a hollow, permanent magnet.

Another object of the invention is to provide a terbium I Faraday rotation glass which is useful at two commonly used laser wavelengths without change of magnetic field and without change of the dimensions of the rotation glass.

A further object of the invention is to provide a Faraday rotation glass of predetermined composition and dimension to provide predetermined rotation of various wavelengths of polarized light by immersing the glass in different temperature 2 DETAILED DESCRIPTION In general, the present invention provides a single optical isolator, generally indicated by reference numeral 10, which glass is immersed in a coolant bath at a predetermined temperature, and a laser light or other polarized or coherent light in a predetennined wavelength range passing through the glass provides a predetermined and variable angle rotation, depending on the temperature of the bath.

The terbium-doped silica glass element 12 is mounted typically in a generally cylindrical enclosure 14 having windows 16 and 18 attached to the ends thereof. The windows 16 and 18 are essentially transparent to light of the wavelengths of interest. A jacket 20 surrounds the enclosure 14 and provides an annular cavity 22 therewith for the containment of the desired refrigerant. A monochromatic beam of light, indicated by arrow 28, from source 30, such as a laser, passes through the window 18, is rotated by the element 12, and exits through the second window 16.

It has been found that the paramagnetic Faraday rotation effect is approximately inversely proportional to the temperature and the terbium ions in the glass produce a rotation which is negative in sign. Further, the Faraday rotation effect almost always increases as the wavelength of the radiation is changed to approach that of the ultraviolet absorption edge. Effective advantage of the Faraday effect is achieved by using light wave radiation which is shorter than the absorption wavelength and directing such radiation through the glass under predetermined temperature conditions. The radiation passing through the glass will be rotated by an angular amount 0, the value of the angle 0 is given by the following equation:

' 0=VHL wherein V is the Verdet constant, H is the applied magnetic field, and L is the length ofthickness in centimeters of the glass element.

As a specific example, silicate glass doped with 58 weight percent of terbium oxide is formed to a length of 8.2 cm. This piece of glass is immersed in liquid nitrogen and a laser beam of 1.06 microns wavelength is passed through the glass. The 1.06 microns is the wavelength of a neodymium laser. The glass is subjected to a magnetic field from a permanent magnet at a l gauss axial field. At these particular parameters, the rotation of the light is 45. With the same piece of Faraday rotation glass at the 8.2 cm. length and with the permanent magnet of the 1100 gauss axial field, a laser beam at 0.6943 microns (the wavelengths of ruby laser emission) is rotated 5 l.5 when the glass is immersed in dry ice and acetone. Since the loss in directivity is proportional to All) the optical isolation is 25 db. at 1.06 microns and will be at least about 20 db. at 0.6943 microns. Also, by formingthe terbium glass to a somewhat lesser thickness, the difference may be split, for example, by forming the glass of a thickness of 7.7 cm. a 25 db. optical isolation may be obtained at either wavelength merely changing the-refrigerant.

The extrapolation or interpolation to other wavelengths is easily. accomplished as there are a larger number of appropriate refrigerants at various'temperatures available. Since the Faraday rotation effect is approximately inversely propor- I tional to the temperature, a single optical isolator may be made which is useful at a plurality of laser or other wavelengths.

lclaim:

l'. A multiple wavelength optical isolator for use with a source of light, the source being selected from the group consisting of materials providing a monochromatic beam of light having a wavelength of 0.6943 pm. or 1.06 mm, the isolator being chosen from the group consisting of liquid nitrogen comprising: and the combination of dry ice and acetone whereby the a piece of high Verdet constant silicate glass doped with first-named refrigerant produces isolation for the from 30 to 70 weight percent of terbium oxide, the m nochromatic eam at the Wa elength of 1.06 pm. and thickness of the piece of glass along the beam being in the 5 a the second-named refrigerant produces isolation for the range of 7.7 to 8.2 m, monochromatic beam at the wavelength of 0.6943 pm.

a permanent magnet having a field strength of approximate- A lflultiple Wavelength P isolator E claim [y 1 100 gauss disposed with its lines of force passing 1, m which the thlcknessof the ptece of glass IS 7.7 cm. and the through m piece f glass na to the beam, and glass is doped with 58 weight percent of terbium oxide.

a refrigerant surrounding the piece of glass, the refrigerant 

2. A multiple wavelength optical isolator according to claim 1, in which the thickness of the piece of glass is 7.7 cm. and the glass is doped with 58 weight percent of terbium oxide. 